The present invention relates to a displacement measuring device for use with small measuring devices such as digital vernier calipers, in particular, to a so-called absolute displacement measuring device for measuring absolute displacement of a movable element against a fixed element.
In small measuring devices (such as digital vernier calipers, digital micrometers, and height gauges), capacitance-type displacement sensors have been increasingly used. A capacitance-type displacement sensor comprises a fixed element (such as a main scale) and a movable element which relatively moves against the fixed element (such as a slider). On the fixed element and the movable element, a large number of electrodes are disposed in respective patterns. As the movable element moves against the fixed element, signals which represent cyclic variations of capacitance created between electrode patterns are detected and thereby a displacement is obtained.
Such displacement sensors are categorized as two types which are incremental type and absolute type which differ in the formats of output signals thereof. In the incremental-type displacement sensor, as the slider moves from a reference position, cyclic signals take place. By continuously measuring the cyclic signals, the displacement is measured. On the other hand, in the absolute-type displacement sensor, the absolute displacement (position) of the movable element against the fixed element is obtained without necessity of the continuous counting operation of the cyclic signals. In the absolute-type displacement sensor, corresponding to the shapes of electrode patterns formed on the fixed element and the movable element, cyclic signals with coarse pitches (coarse scale signals), cyclic signals with medium pitches (medium scale signals), and cyclic signals with fine pitches (fine scale signals) are output. By composing phase information of these cyclic signals, an absolute displacement of the movable element can be detected. The theory of such a capacitance-type absolute displacement sensor is disclosed in for example U.S. Pat. No. 4,879,508 and the like.
In the absolute-type displacement measuring device, the above-described coarse scale signals, medium scale signals, and fine scale signals are demodulated as square cyclic signals with phase information at their edges. When the phases of these cyclic signals are detected, the phases of leading edges and trailing edges of the cyclic signals are detected several times and then averaged. This is because these operations can prevent the phase information from being affected by the offset of an operational amplifier of a demodulator and display values from fluctuating due to data variation.
When the slider of the displacement sensor relatively moves against the main scale (this operation is referred to as the movement of the displacement sensor), the period of each signal obtained by the demodulator is prolonged or shortened. The degree of the variation of the signal period is varied depending on the moving direction of the displacement sensor. This is because although the frequency of excitation signal of the capacitance-type displacement sensor is constant, the movement of the displacement sensor causes the output signal to frequency modulate depending on the variation of the opposed relation of transmitter electrodes and receiver electrodes. The variation of the signal period is large in the order of the fine scale signals, medium scale signals, and coarse scale signals. Since the signal period is varied, the period for sampling phase data varies depending on whether the displacement sensor stops or is moving.
Thus, the period for sampling phase data (by detecting leading edge and trailing edge) in the condition that the sensor is moving is longer than that in the condition that the sensor stops.
As described above, in the absolute-type displacement measuring device, when the displacement sensor moves in a particular direction at high speed, sampling of necessary phase information takes a long time. If an analog circuit operates for sampling phase data in such a long time, the power consumption of the device would increase. If the display refresh cycle is varied according to the sampling period of phase data, the phase data could not be regularly displayed. If the display refresh cycle is set too long in consideration of the variation of phase data sampling period, data would not properly follow the movement of the sensor and thereby the display thereof would be unnatural.